1. Field of the Invention
The present invention relates to the bonding of expanded polymeric parts, and, more particularly, to an improved method for bonding expanded polymeric parts together by irradiating a non-aqueous, adhesive-free dielectric bonding composition interposed between the parts to be bonded with application of high frequency electromagnetic energy.
2. Description of Related Art
Industry commonly uses foam parts as patterns to form vaporizable molds which are then used to cast metal shapes in the "lost foam process". The lost foam process involves embedding a vaporizable pattern in sand, which then fills the hollows and recesses of the pattern. The pattern is typically made of vaporizable expanded polymeric parts, such as polystyrene, and is produced using conventional molding techniques. A molten casting material is poured into the embedded pattern, whereupon the heat of the molten material vaporizes the polymeric material making up the pattern. The gases produced upon vaporization escape into the surrounding sand. The result of this process, once the casting material cools and solidifies, is a casting in the shape of the pattern.
The automotive industry in particular uses the lost foam process to cast large and complex engine parts such as heads, blocks, and manifolds. Given the size and complexity of the vaporizable patterns required to yield such castings, the vaporizable patterns are commonly produced in a piecemeal fashion. Instead of attempting to produce a complex pattern in a single molding the patterns are produced by bonding together easily molded components. While the piecemeal approach simplifies the molding process, the bonding step introduces a new set of concerns to the lost foam process.
At least three techniques have commonly been used to bond expanded polymeric parts for use in the lost foam process. In one technique, a hot melt (thermo-plastic) adhesive is applied to the surfaces of the parts to be bonded. The parts are then mated prior to the cooling of the adhesive to achieve an effective bond, a step accomplished in automated processes with the use of specially designed fixtures. This technique is inconvenient for several reasons. First, the hot melt thermoplastic tends to flow into unwanted places on the polymeric parts as well as the surrounding work area, creating substantial messes and requiring costly clean up efforts. Second, a change in the shape of the parts necessitates a costly change in the fixtures, since the fixtures must mirror the configuration of the parts to be bonded. In addition to being inconvenient, this technique can damage or deform the polymeric parts by subjecting these parts to chemical attack from certain adhesive compositions.
In another technique, water is used to bond the parts rather than an adhesive. For example, U.S. Pat. No. 4,035,216, issued Jul. 12, 1977 to Richard H. Immel, discloses a method for bonding lost foam patterns together by wetting one or more surfaces to be bonded with a water/detergent mixture and subjecting the patterns while in a mated relationship to a high frequency electromagnetic energy. The polymeric parts are effectively transparent to the high frequency electrical energy, but the water serves as an electromagnetic susceptor compound, thereby absorbing the electromagnetic energy and converting it to thermal energy. As a result, the thermal energy heats the mated surfaces of the polymeric parts, causing them to melt and expand into each other to create a bond. The detergent is present at a concentration of up to about 2% to improve the wettability of water on the surface of the polymeric parts.
Using a water/detergent mixture to bond polymeric parts invites several problems. First, during casting, water remaining in the polymeric parts forms hydrogen, which in turn creates unwanted porosity in the metal castings subsequently produced from the lost foam process patterns. This porosity can dish the mechanical strength of the castings as well as mar their surface finish. Second, detergents introduce a release layer on the surfaces of the polymeric parts that impedes later efforts to perform secondary bonding or coating operations on the joined polymeric parts.
Finally, several patents describe a technique for bonding wherein an adhesive composition is cured with high frequency electromagnetic energy. For example, U.S. Pat. No. 4,253,898, issued Mar. 3, 1981 to William R. Rinker et al, describes a vinyl plastisol composition for microwave bonding of plastic parts. This process is dependent upon the melting of the plastic parts, which can deform the lost foam component and result in a mis-shapened metal casting. Additionally, the mere application of adhesive compositions on the polymeric parts can be damaging, since adhesive compositions may chemically attack the foam. Finally, the use of adhesive compositions commonly generates substantial messes requiring costly clean up efforts.
In another example using adhesive compositions, U.S. Pat. 4,018,642, issued Apr. 19, 1977 to Robert L. Pike et al, describes the use of phenolic resins in the microwave bonding of wood for the production of plywood type materials. As discussed regarding Rinker et al above, adhesive compositions in general can damage the plastic parts by chemical attack and can result in wasted time and effort expended toward cleaning inevitable spills and messes. Moreover, by employing phenolic resins in a casting process, one risks generating an undesirable residue in the casting. Notably, Pike et al make no inference with respect to the utility of bonding expanded polymeric parts with this technique.
Thus, a need exists for an improved method for bonding expanded polymeric parts for use in the lost foam process that avoids the above-mentioned problems and inconveniences wrought by using water or adhesive compositions in bonding.